The tool bag dropped in space by a US astronaut has joined hundreds of thousands of other pieces of rubbish orbiting Earth.

The European Space Agency estimates that there are more than 600,000 objects larger than 1cm in diameter floating in space, while America's Space Surveillance Networks regards the 12,500 10cm-sized objects large enough to warrant constant monitoring.

Because space debris can reach speeds of more than 20,000 miles per hour, scientists and space authorities fear that serious damage could be caused to shuttles or stations.

Another potential danger was added to the list this week when Heidemarie Stefanyshyn-Piper watched her tool bag drift away during a spacewalk in order to repair a part of the international space station.

It joins some other improbable items among the collection of space rubbish. A golf ball is still floating somewhere in space after it was hit from the international space station by Michail Tjurin, a Russian astronaut, as part of an advertisement for a Canadian sports company in 2006. Several astronauts have lost their gloves during space walks.

Items lost by other astronauts during their time in space also reportedly include a camera, wrench, screwdriver, nuts and bolts, cloths, glue guns and a robotic arm.

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Meanwhile, about four items of junk are released into space each year by the international space station - a significant improvement on the old Mir space station, which used to dump 22 packages of rubbish each year.

More conventional items include the outer section of the Sputnik rocket, which was the first piece of rubbish to be dumped into orbit after its launch in 1957. Nuclear reactor cores from former Soviet military satellites also orbit Earth, while experts are particularly concerned about parts of rockets that are floating while laden with unused fuel, which could cause explosions.

More than half of the 6,000 man-made satellites that have been launched into space are also still in orbit, despite only about 800 of these remaining operational.